Isn't it just a matter of removing filler and drain plugs in that order, ideally after a good run to warm oil. On my axle the filler plug required a hex key and drain plug required a socket for hex head. Obviously have a tray underneath to catch oil with minimum 2 litres capacity.
I leave mine to drain fully maybe even overnight, then replace drain plug and refill with recommended oil through filler plug until it just starts to gently dribble out of plug hole, then replace. Access from above after removing tool tray can help.
For reference the BTR axle on my Roadster took 1.6 litres.
